Printing inks and printing methods may be broadly divided into four major classes which vary considerably in ink compositions, method of applications and drying mechanisms. Typographic printing is a method of printing from raised characters or plates which are first inked and then pressed against the surface to be printed, thus transferring the ink. The ink must have sufficient viscosity (tack) and adhesion so that it can be evenly distributed on the rollers of the press and be transferred properly from the type on printing plates to the paper or other substrate being printed.
The second major method of printing is lithography which is also known as planographic printing since the process prints from plane surfaces and depends on the fact that grease and water are mutually repellent. If a design is drawn with a greasy substance on a finely-grained metal surface which retains moisture, the lithographic ink will adhere to the design and not the remainder of the plate. In the form of offset lithography, the greasy image is formed on a thin grained metal plate by photographic means and the metal plate is curved and clamped around the plate cylinder of an offset press. Lithographic or offset inks are more viscous than typographic inks and must be substantially resistant to emulsification due to the high speeds developed by offset presses, the presence of water and the resulting high rates of shear. Additionally, the presence of large amounts of volatile solvents in the ink vehicle is undesirable since the solvent will tend to dissolve away the greasy image and cause the plate to go blind. Evaporation of the solvent further gives rise to increased viscosity of the ink which, in time, affects the fidelity of printing.
The present invention is concerned with the third and fourth major methods of printing, i.e., gravure printing and flexographic printing. Photogravure, rotogravure or gravure printing (the three terms will be used interchangeably) is a generally-known printing technique whereby intaglio engravings of an image to be printed on a substrate (usually paper) are formed on the surface of a grooved cylinder. Intaglio engravings are those where the elements to be printed are formed below the surface of the gravure cylinder, typically by engraving into a metallic cylinder, thereby forming ink-retaining grooves or cells in the cylinder. Used in conjunction with the etched or engraved gravure cylinder is the very important doctor blade which controls the amount of ink transferred from the printing surface of the cylinder to the paper or other substrate material.
In rotogravure intaglio inks which are used to print at high speeds from chemically or electronically engraved copper or chromium-faced cylinders or anilox rolls, the wiping of the cylinders or rollers is achieved mechanically by means of a doctor blade. A doctor blade is usually a long, thin resilient strip of metal that is mounted in the printing machine parallel to the axis of rotation of the gravure cylinder or anilox rollers, and the edge of the doctor blade comes in contact with the surface of the cylinder or the anilox rollers. When the machine is in operation, the doctor blade controls the amount of ink transferred to the substrate by wiping the excess ink from the surface of the cylinder or anilox rollers, thereby leaving only the ink within the etching or engravings for transfer.
While proper etching or engraving of the cylinder or anilox rollers is extremely important to the quality of the ultimate printed product, the significance of the doctor blade cannot be discredited. Indeed, even if the cylinder or anilox roller has been properly engraved, excessive or uneven doctor blade wear and improper usage will prevent the obtaining of a proper printed image.
By and large, doctor blade assemblies are a two-part configuration of a back-up blade and a doctor blade. Both the back-up blade and the doctor blade are held in a blade holder so that the doctor blade is urged against the cylinder. Even though the doctor blade will wear as a result of the wiping action against the cylinder and must eventually be replaced, the back-up blade is usually reusable. The back-up blade adds support to the very thin doctor blade and securely holds the thin doctor blade in the block holder.
In flexographic printing, also known as relief printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate web, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
Japanese Laid-Open Patent Publication No. 60-238,375 (Honda et al., Publication Date Nov. 27, 1985) relates to a water-based ink composition for ball point pens, comprising as essential components a dithiophosphate represented by the following general formula, a coloring agent and water: ##STR2## wherein R is a hydrogen atom, a lower alkyl group or an alicyclic hydrocarbon group; X is an alkali metal atom, an alkaline earth metal atom, ammonia, an N-substituted ammonium or piperidinium.